Page 187 - Basic Well Log Analysis for Geologist

JIG INTERPRETATION CASE STUDIES

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Case Study 3 Answer
An evaluation of the logs begins with an assessment of A crossplot (Fig. 82) of irreducible water saturation (Sy
sand development in the well through the Wilcox interval. ir) Versus porosity (db) reveals a permeability range of 10 to
because this interval is where a good gas show appeared. A 50 md. You conclude, therefore, that the reservoir has good
porous and permeable sand is read on the SP log (Fig. 79). permeabilities.
from 10.924 to 10,982 ft, by a leftward deflection of the Data on acrossplot (Fig. 83) of irreducible water
curve froin the shale baseline. saturation (S,, ;,,) versus water saturation (S,,). are plotted
The R,,, quick look curve (Fig. 79) deflects to the right, in arange from intermediate to low relative permeabilities
away from the SP curve, in the upper part (10.935 to 10,958 to gas (K,,). And, those data points which have low
ft) of the Wilcox sandstone. Such a curve deflection ts permeability to gas values (fess than 10%) are from the
evidence of the presence of hydrocarbons. lower Wilcox zone. As K,, values decrease, relative
On the deep induction log (Fig. 79), reading R,, permeability to water increases and the reservoir will
resistivities Increase in an Upper Zone covering produce some water. In the case of your company’s Wilcox
approximately 10.924 to 10.958 ft. Increased resistivity well. because of the position of the lower zone’s data
also indicates hydrocarbons. points, the amount of water produced may be appreciable.
A fairly rapid decrease of resistivities into the lower zone On the crossplot (Fig. 83) data points with values ef relative
and a deflection of the Ry, curve to the left at 10.958 to permeability to gas which are greater than 10% should
10,982 ft means higher water saturations. These produce decreasing amounts of water. These data points are
observations alert you that the lower zone may be water from the upper zone.
productive. The moveable hydrocarbon index (S,/S,,,) 1s less than 0.7
In order to establish whether or not the lower Wilcox trom 10.924 to 10.958 ft. indicating hydrocarbons have
zone will produce water, you decide that a rather detailed moveability. The high moveable oil saturation values
log evaluation is necessary. The evaluation will include a (MOS) also suggest hydrocarbons will move.
Pickett crossplot for water saturations, Archie equation The lower bulk volume water values (Fig. 84) arid lower
calculated water saturations and crossplots for grain size. water saturations in the upper Wilcox sandstone. when

relative permeability, permeability. and bulk volume water. compared to the lower, indicate only the upper Wilcox may
A Pickett crossplot (Fig. 80) of deep induction log be above a gas/water transition zone. The bulk volume
resistivities (R,) versus sonie log interval transit (At) time, water values which are much greater than 0.1 (Fig. 84) are
corrected for a sand matrix (At — At,). has water from the lower part of the Wilcox from 10,958 to 10.982 ft.
saturations ranging from [00% to less than 50%. High This interval is above irreducible water saturation.
water saturations of greater than 71% on the Pickett All of the data produced by your evaluation support your
crossplot are from the lower Wilcox zone. This supports early assessment of a reservoir with gas on top of water. You
your previous suspicions about its water productive nature. were immediately alerted to a potential problem after you
Examination of the sonic log reveals porosities of 14 to examined the Ry, quick look curve and saw a deflection to
20% after correction tor gas effect. These calculated sonic the left through the lower Wilcox zone, and when you saw
porosities are used in the Archie water saturation equation the fairly rapid decrease of resistivities into the lower zonc.
(Sq) and in making crossplots. An estimate of recoverable reserves by volumetric
A determination of Wilcox sandstone grain size as very calculations of the Wilcox sandstone is 2.3 BCF using the
fine-grained is made after you review a crossplot (Fig. 81) following parameters: drainage area = 240 acres: reservoir
of water saturation Archie (Sy..) versus sonic porosity (A). thickness = [5 ft: porosity = 18%; water saturation = 57%:
High water saturations of 70 to 80% in the lower zone gas gravity (estimated) = 0.62; recovery factor = 0.6:
(10.958 to 10.982 ft) probably invalidate grain size temperature (estimated) = 279°F; initial bottom hole
determinations, because the reservoir ts not at irreducible pressure (IBHP, estimated) = 8.103 PSI: Z factor = 1.229:
water saturation (Sy. jpp). geothermal gradient = 0.0255 x formation depth: and
Water saturations of the upper part (10,924 to 10,958 ft) pressure gradient = 0.74 x depth.
of the Wilcox range from 43 to 62% (a relatively high You estimate the return on investment of the Wilcox
level), and result from the very fine-grained size of the sandstone as follows:
Wilcox sand. The relationship of grain size and water 1. Total Well Cost x Working Interest = Working
saturauion ts such that fine-grained sands have high Interest Well Costs: $1,800.000 x 0.25 = $450,000.
irreducible water saturations (see Chapter VI), and 2. Net Revenue Interest (lease) x Working Interest =
consequently they also have high values for bulk volume Net Revenue Working Interest: 0.825 « 0.25 = 0.20625
water. (20.6%).

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